Conventionally, LSIs are formed by integrating elements in a two-dimensional plane on a silicon substrate. Although the size of one element is ordinarily reduced (miniaturized) to increase the storage capacity of a memory, this becomes recently difficult from a viewpoint of cost and technology. Although a photolithography technology must be improved for miniaturization, a cost necessary for a lithography process is more and more increased. Further, even if miniaturization has been achieved, it is predicted that a withstanding voltage between elements and the like reaches a physical limit unless a drive voltage and the like are scaled. That is, there is a high possibility that a device becomes difficult to operate.
To cope with the above problem, recently, a lot of semiconductor storage devices are proposed in which memory cells are three-dimensionally disposed to increase the degree of integration of the memories (refer to Japanese Patent Application Laid-Open No. 2007-266143 and U.S. Pat. Nos. 5,599,724 and 5,707,885).
As one of conventional semiconductor storage devices in which memory cells are disposed three-dimensionally, there is a semiconductor storage device using a transistor having a columnar structure (refer to Japanese Patent Application Laid-Open No. 2007-266143 and U.S. Pat. Nos. 5,599,724 and 5,707,885). The semiconductor storage device using the transistor having the columnar structure is provided with a multi-layered conductive layer acting as a gate electrode and a pillar-shaped columnar semiconductor. The columnar semiconductor functions as a channel (body) of the transistor. A memory gate insulation layer is disposed around the columnar semiconductor. An arrangement including the conductive layer, the columnar semiconductor and the memory gate insulation layer is called a memory string.
In the above conventional technology, holes are formed to the laminated conductive layers at the same time. Subsequently, memory gate insulation layers are formed to the side walls of the thus formed holes and subjected to a diluted fluorinated acid process. Then, columnar semiconductors are formed so that the holes are filled therewith. The memory cells are three-dimensionally formed by repeating the above processes a plurality of times. However, a problem arises in that the memory gate insulation layers are removed by etching due to the diluted fluorinated acid process.